Design of Offshore Concrete Structures _ch02

Design of Offshore Concrete Structures _ch02 Written by experienced professionals, this book provides a state-of-the-art account of the construction of offshore concrete structures, It describes the construction process and includes: *concept definition *project management, *detailed design and quality assurance *simplified analyses and detailed design

2 Concept definition and project organization Ove T.Gudmestad, Statoil 2.1 Objectives The objectives of Chapter are to contribute to: • • • • give an overview of the requirements for design of offshore concrete structures convey the experiences from prior projects, to those having special interest in offshore concrete structures promote and enhance the confidence in offshore concrete structures give an overview of how to design a concrete platform, an overview which can also be suitable reading for students 2.2 General description of an offshore concrete structure Prior to any further discussion regarding design of an offshore concrete structure, references are made to Figures 2.1 and 2.2, which show typical fixed and floating concrete structures, respectively It is of special importance, for further insight, to recognise the names of the various elements of the structures For several typical offshore concrete concepts, floating stability is not achieved if one (or more) of the compartments are damaged and flooded with water This is representing a line of thinking in design which is not common in connection with ship-design It also means that structural design must be done with particular care For fixed bottom founded concrete structures the importance of floating stability applies during the floating phases only, as the structures cannot sink after being installed offshore Floating concrete structures have to be designed with sufficient safety against sinking, in case compartments facing open sea would be filled with water during operations at the field For design of concrete structures the requirements of Section 18 of the Norwegian Petroleum Directorate’s “Regulations relating to load bearing structures in the petroleum activities” should be given special attention: The structural system, details and components shall be such that the structures: a) b) c) d) e) f) show optimum ductile properties and little sensitivity to local damage are simple to make provide simple stress paths with small stress concentrations are resistant to corrosion and other determinations are suitable for simple condition monitoring, maintenance and repair are removable © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin Fig 2.1 Gravity Base Structure (Gullfaks C platform in North Sea) © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin Fig 2.2 Tension leg platform © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin 2.3 Project phases During design of an offshore structure it is worthwhile noticing that the work is performed in several project phases with an increasing degree of detail (Fig 2.3) During the first phase, for example, the advantages of various structures is assessed, and comparisons are made for field developments using various types of structures As part of the work during the detail design phase, which forms part of the construction phase (not shown in Fig 2.3), the detailed calculations are made For concrete structures this includes geometry drawings, rebar drawings, rebar bending schedules, etc More detailed description of the work in the various phases are given in the following sections; see also Fig 2.3 and Appendix A Fig 2.3 Project Phases for Design of Marine Structures 2.4 Rules and regulations Offshore concrete structures are to be designed according to national rules and regulations (see Section 1.6 and also (NPD, 1992), (NBR, 1998) and (NBR, 1999) © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin 2.5 Project management 2.5.1 Project planning (a) The objective of project planning Design of an offshore structure should be regarded as a project, i.e a set of tasks to be accomplished within a specified period of time, and with limited resources Also, there must be a project organisation with responsibility for execution of the project task A project is a link in a chain, where the effectiveness and quality, among other things, depend on the interaction between the various links; project employer, project and supplier, Fig 2.4 Fig 2.4 Description of a project as a link in a chain The purpose of the project planning is thus to: • • • • distribute responsibility, authority and tasks achieve high quality of the project results manage resources, time and cost and control the use of them reduce the use of double work and unproductive/unnecessary project tasks (b) Control activities To achieve the objectives of the project planning, it is important to establish necessary control activities to ascertain the fulfilment of the objectives (Fig 2.5) Fig 2.5 Control activities © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin The control activities are: • • • • • • to establish goals establish an activity plan to reach the goals control the execution of the project in accordance with the plan follow up the execution identify and analyse plan deviation plan and perform improvements and, if necessary, take care of corrective activities Design of offshore structures will be a sub-project within a major investment project An investment project can be characterised by a high exposure of cost, combined with high uncertainties The uncertainties are partly linked to the investment cost for facilities and partly to future incomes The development of an investment project will last for years, with several decision points (milestones) The project is therefore sub-divided into project phases as discussed in the previous sections of this chapter 2.5.2 The project control basis (a) Introduction The project control basis, Fig 2.6, can, as a minimum, be defined as: • • • • work scope activity plan (network) with planned progress cost estimate (time distributed costs) authorisation Fig 2.6 Project control basis © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin The control basis should be compiled before the start of each of the phases in the investment project In addition, the project control basis should define the control parameters influencing the project objective The control parameters should be consistent through all the project phases, and should be updated when new information gives grounds for changing the parameters The result of the planning process: milestones, resource planning and cost phasing establish an execution plan as control basis for the next project phase (b) Project breakdown structure The project control basis should be broken down according to a standard cost coding system, enabling easier planning and control of the project, such that deviations can be detected and corrective actions implemented The cost coding system should make allowances for various requirements, depending on the project phase, i.e if it is in an early planning phase or in a later project phase (execution) The cost coding system is designed such that planning data for various project alternatives can be compared and analysed in all the project phases The cost coding system will be the foundation for systematically feeding back of experience data and for compilation of time schedules and estimates The cost coding (Fig 2.7) accommodates the following three hierarchy structures: • Physical Breakdown Structure • Standard Activity Breakdown • Code of Resource Fig 2.7 Standard cost coding system The combination of physical extent, standard activity and resource type gives the foundation for a standard preparation of plans, cost estimates and experience data The work scope is broken down into work packages (Fig 2.8) in the execution/development phase © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin Fig 2.8 Work breakdown structure (AFE= Authorization for Expenditure) The breakdown into work packages should take the following into consideration: • • • • • • organisation and ownership contract philosophy supplier marked availability work complexity interface internally and externally in the project method of assessment and control of workmanship In the concept definition phase the cost coding of the control basis, in accordance with the Standard Cost Coding System, should be carried on from the project development phase During the execution phase, the project control basic is structured in work packages The control basis is broken down into a level below work packages (planning level 3) The project defines requirements to suppliers’ systems The requirements should be related to the interface between the project and additional vendors, enabling the individual vendors to use their own systems The control basis should be possible to aggregate on all levels (c) Execution plan Execution plans for the project shall be prepared with relations and limitations to: • time • resource • cost Detailing of the execution plans is dependent on the level of ambitions and on requirement with respect to uncertainties The execution plan is part of the project agreement between client and project, and relates to the project’s main plan, which forms the basis for project development decision At all times must progress, milestone achievements and other activities during execution of the project be related to the project’s main plan The execution plans are thus important references for control of the project © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin The execution plans should include: • • • • scope of work (including technical specifications) progress plans (including externally given milestones) resource plans cost estimates (including budgets) The relation between scope of work, time, resources and cost are linked to the lowest level (planning level 0) in the project’s Work Breakdown Structure (WBS) see Fig 2.8 (d) Scope of work The client is responsible for a proper definition of the project’s goals, and to ensure that the goals are understood by all parties involved The main goal of a project is always to strive for cost/benefit-effect (i.e to maximise the profit on the invested capital) The correlation between the various sub-goals for the project and the main goals can be difficult to understand The project control parameters must therefore be clearly defined, to assure that all involved have got a mutually agreed understanding of common goals, project tasks, assumptions/ frame conditions in the entire chain from client to project and to contractor/supplier Project agreement The project goal and the overall control parameters shall be documented in a project agreement The project agreement shall describe goals and tasks, assumptions and frame conditions, plans and estimates, responsibilities and authorities The document is prepared by the client Contracts The need for mutual goals and understanding of project scope, assumptions and frame conditions also applies to the supplier for those parts of the project for which he is being given responsibility During contract formulation (see also Section 2.8), and following-up of the contract, it must be assured that the project’s requirements to management and control systems is met so that project goals can be reached By setting contract requirements for quality management and control to contractors/ suppliers, the possibility of preventing negative deviations are increased (e) Schedule The overall progress plan forming the basis for execution of the project, is called “Master Control Schedule” During project execution, deviations will occur, hereby creating the need for schedule revisions, called Current Control Schedule The work packages in the project containing volume, time and cost shall be split into work orders, CTRs (cost, time, resource estimates), by the contractor/supplier Schedules are normally presented in two ways: A network (Fig 2.9) containing necessary information about work sequences and logic for the aim of analyses, as well as a Ganttdiagram for presentation purposes of the project, (Fig 2.12) © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin Network The activities’ dependence on each other should be modelled in a project network (Fig 2.9) The level of detailing and complexity in the network model will be determined by the project’s complexity, magnitude and requirements for quality and follow up The network definition comprises of: • • • • activity dependence with type of bonding early start/finish late start/finish delays/overlaps Fig 2.9 Project network Analysis and presentation (Gantt-diagram) The final schedule, with built in slack and overlapping activities, should be drawn up and determined from what will overall give the best project economy The likelihood of meeting the ending date or in-between milestones should also be determined (Fig 2.10) Fig 2.10 Analysis of progress schedule © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin The estimation norms are so set that under given circumstances there should be equal probability for result over as under the individual unit rate (50/50 estimate) An estimate is presented with an expectation value (50/50 estimate, i.e the value giving the same probability for over/under-run), min/max values and confidence level All four variables; scope, complexity, productivity and price are related to uncertainties and they should, dependant on method used when estimating, interpretation of available data, etc., be described by a probability distribution (Fig 2.14) Simplified, this can be a 50/50 value in addition to the low/high values Fig 2.14 Cost estimation uncertainty Requirements for cost estimation and schedule classification Requirements for cost estimation and time scheduling classification is a classification system with defined requirements to: • • • • • • basic information, work scope estimation method level of detailing time scheduling uncertainties analysis, etc presentation and documentation formats The classification requirements shall describe the method for cost estimation and time scheduling, give requirements to technical information needed to perform the planning and the need for the accuracy of the estimate (Fig 2.15) © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin Cost estimates are refined during the course of the project to reflect the additional detail available A progression of five types of construction cost are normally used; order of magnitude, conceptual, preliminary, definitive and control Fig 2.15 Cost estimate in different phases (h) Risk assessment Project risks shall be identified, analysed and responded in order to maximise the results of positive events and to minimise the consequences of adverse events Risk identification consists of determining which risks are likely to affect the project and documenting the characteristics of each In project context, risk identification is concerned both with opportunities (positive outcome) as well as threats (negative outcomes) The identified risk items shall be quantified to assess the range of possible project outcomes Risk response development involves defining enhancement steps for opportunities and responses to threats (i) Budget Budget Estimate A budget is established administratively through a management decision The basis for the decision can be a cost estimate, but the budget itself is not linked with uncertainties The budget is a known, deterministic figure The budget may have different meanings: • a cost frame for the project to be kept within • an expression of the project’s expected total cost • a target figure for the project organisation to reach at The different understandings of the budget reflect the extent of authority the project has got in spending money © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin Project budgeting The budget (Master Control Estimate) is normally equal to the project’s expected cost (50/50-estimate) at the start of the project (Fig 2.16) The budget is not changed during the execution of the project unless agreed changes between client and project to scope of work or conditions are implemented Budget changes are always made based on chosen standards of estimation from the original budget Fig 2.16 Project budget 2.6 Work during early phases of a project 2.6.1 Early phase activities In the early phases (i.e Phases and as shown in Fig 2.3), the basis is prepared for a good and sound field development solution Work during the early phases are to ensure: • that there is an economical potential to continue the project into later phases • that the technical solutions are robust enough to ensure that minor changes in the design basis, or minor inaccuracies in structural design not lead to large increases in volumes, dimensions or costs • that the technical solutions actually dealt with, are feasible within the given budget Note that the uncertainties in the cost estimates are meant to be gradually reduced during the consecutive stages of the early phase work The work in the early phases covers the following phases of a project, (Fig 2.3): Phase Exploration Prospect evaluation Objective: Develop basis for Application for Concession Field evaluation Objective: Identify a combined technical and commercial solution © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin Phase Project development Feasibility study Objective: Reach a decision whether the actual field is commercial and prepare a Report of Commerciality Field development study Objective: Describe the best economical solution for field development, and prepare a Plan for Development and Operation (PDO) of the field In Norway This PDO-document is to be submitted to the authorities for evaluation and for final approval in the Parliament During the early phases the possibilities of influencing the development solution and the economical results of the final product is high Work done in later phases tends more to focus on details, and the extent of documentation increases It is important to put forward requirements of what efforts are needed to ensure that enough work is performed, such that: • the results satisfy the requirements of the phase in question • the results are sufficient to start the work in the next phase As a result of this, a Discipline Activity Model, which describes the required extent of work in the various phases of the study, will be useful A typical Discipline Activity Model for an offshore concrete structure is included in Appendix A Note that the Appendix defines the need for analysis in the various phases of the project Furthermore, it defines how the work shall be quality assured The Appendix also specifies which reports are to be issued during the various phases In all phases, a technical basis shall be developed to serve as a basis for estimates of costs and plans The technical basis has to build on realistic information about field parameters Of particular importance for production are parameters like: • production volume • number of wells, risers and J-tubes for pull-in of production pipes from subsea wells and from other fields • weight and layout of the production plant (topsides) • required storage volume Furthermore, field specific environmental parameters are important for assessment of the field development solution Of special importance is information regarding: • • • • • water depth wave height and sea current earthquake conditions (in areas with potential earthquakes) ice conditions (when relevant) geotechnical conditions © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin Fig 2.17 Input to cost estimating programme for estimate in the prospect evaluation phase of the development © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin Fig 2.18 The design process is an iterative process © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin 2.6.2 Selection of concrete structures Concrete structures are relevant for quite a number of various field developments It should be noted that: • fixed concrete structures with short skirts were developed especially for the hard seabed conditions and the large production plants needed for the North Sea during the 1970’s to 1980’s • fixed concrete structures with long skirts were developed especially for the soft soil conditions into and in the Norwegian Trench (Tjelta and others, 1990) • floating concrete structures have been developed over a long period in the 1980’s and 1990’s A concrete tension leg platform with concrete foundations have been installed at the Heidrun field For oil production at the Troll field a concrete floater with chain moorings have been installed 2.6.3 Design of concrete structures in the early phases, examples In the Prospect Evaluation Phase, data gained by experience are used to evaluate the potentials of new blocks The work can be simplified by the use of PC-based tools An illustration of input data needed is shown in Fig 2.17, see also Appendix A (Discipline Activity Model for Design of Offshore Concrete Structures) In the Field Evaluation Phase there is a search for one development solution which shows profitability Even if detailed technical studies are not performed, it is of importance that the persons performing the job have enough experience such that: • the actual solution is prepared thoroughly to assure that the cost estimate is realistic • the actual solution has not got too many extra reserves built in such that the economical potential is lost and further work is stopped In the Feasibility Study Phase the technical solutions shall be ranked Uncertainties in the cost estimates are to be reduced to ± 30% In this phase concrete structures are compared with alternative solutions This represents a special challenge to the concrete designers and forces them to establish innovative solutions Both oil companies, relevant engineering groups and contractors are to be involved Notice that the development work to obtain a competitive concept is interactive, as shown in Fig 2.18: • constructive solutions are chosen • load calculations are made and structural analyses are performed • structural design is carried out This is done for the various activities of a development project, including: © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin • • • • • construction transportation installation operation removing Since the objective of the Feasibility Study Phase is to rank the actual technical solutions, the technical work has to be performed thoroughly In particular the need for Quality Assurance is emphasised This is done by engaging highly experienced persons to participate in the studies and to evaluate the technical reports In the Feasibility Study Phase it is also relevant to consider new concepts with large potential, which so far only have been developed to a lower degree of detail Special programs for technology development can be initiated to qualify development solutions with much potential In some countries where the technology development not yet has reached Western European levels, there is particularly large interest in offshore concrete structures, due to the fact that concrete structures can be built by means of: • local resources (sand, cement, rebars etc.) • extensive use of local work force Furthermore, the construction of concrete structures may lead to: • development of local infrastructure (production of cement and reinforcement) • development of engineering companies and technical know-how In a Field Development Study the work in the Feasibility Study Phase is carried to a higher detailed level A more comprehensive technical study is required, to ensure that the cost ~20% accuracy) submitted to the authorities, and which also forms the basis for the estimates (~ authorities’ decisions, are valid also for the successive phases of the development Appendix A (Discipline Activity Model for Design of Offshore Concrete Structures) indicates the level of detail for technical studies relating to an offshore concrete structure Special attention shall be made to the fact that an acceptable Plan for Development and Operation (PDO) which is submitted to the authorities after the finalisation of the Field Development Study Phase, requires good insight in: • the environmental loads and other loads like ship collisions, etc • the geotechnical conditions • the interface between the substructure and the production plant (topsides), including the decision of whether transfer of the topside to the platform shall take place inshore or be lifted on at the offshore site To ensure that the correct concept is chosen also implies that: © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin • choice of concrete quality must be made (to assure a robust structure it is suggested that normal density concrete of quality C70 represents the upper limit for this stage) • choice of routing of flowlines from wells, risers and J-tubes Large risers for gas are generally routed inside a dry shaft or outside the structure until a location below waterline • development of criteria for potential use of jack-up drilling platforms must be made This decides the layout of the foundation of the concrete structure Furthermore it shall be noted that tank-testing can be relevant at this stage, to assure the quality and the feasibility of the structure, see also NPD’s “Regulations relating to loadbearing structures”, paragraph 30 In case the PDO-report does not open up for choices of concrete structures, it will normally not be relevant to suggest that concrete structures should be chosen in the later phases of the field development 2.7 The concept definition phase 2.7.1 Concept report The goal of the concept definition phase (the concept evaluation phase) is to work out a Concept Report, which in turn forms the basis for the detail design and the fabrication contract The concept report shall describe the concept in sufficient details to avoid major changes in the successive detail design phase Furthermore, the concept shall be robust enough to accommodate minor technical changes in the design basis The concept shall also have built in a certain “forgiveness” to allow for minor inaccuracies It is, furthermore, needed to emphasise that weight control shall be an important parameter for those working with the topsides, and major changes in the topsides weight must be avoided For concrete structures it is also of importance that uncertainties in weight, due to the amount of reinforcement and to water absorption in concrete, are included in the concept definition phase This is to assure that the platform has acceptable floating stability during all temporary phases, and for floating concrete structures also during the operation phase The concept report shall also describe important principles for the detail design, and these principles are assumed to be unchanged throughout the construction phase For instance, the extent of vertical post tensioning of the concrete shafts represents a principle which is defined during the concept definition phase Other principles of importance which are suggested to be included in the concept definition phase are: • that the geometry of the concrete is correct Changes to the geometry (e.g wall thickness) could lead to large consequences in the detail design phase • that the concrete quality C70 (for normal density concrete) or LC60 for LWA-concrete (light weight aggregate concrete) are the maximum values applied during the concept definition phase This will give the project some reserves in highly stressed areas It is, for example, particularly unfavourable if the amount of reinforcement is significantly increased during the later phases due to the fact that shell walls are too thin in the early phases It is also very costly © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin to build a structure where there is barely room enough for placing the required amount of rebars as rebar placement and compaction of the concrete then would become very demanding • that all openings towards the sea have double barriers in case of damage • that the lower dome of the drillshafts shall not be penetrated if a conductor-pipe is lost during the construction phase This applies to the cases when the structure is floating with no damage stability and the lower dome is exposed to dropped objects • that (during the concept definition phase) it should be aimed at avoiding membrane tension in the concrete walls throughout the section when the structure is to be exposed to the design wave (ultimate limit state with load coefficient of 1,3) This will contribute to a robust structure with only minor cracks and an acceptable fatigue life 2.7.2 Technical work The technical work during this phase will be comprehensive, as indicated in Appendix A (Discipline Activity Model for Design of Offshore Concrete Structures); see also (Fjeld and Morely, 1983) and (Tjelta, Aas, Hermstad and Andenæs, 1990) Some special problems which shall be evaluated, are as follows: • load analysis with assessment of whether special phenomena may be present due to the geometry of the structure • the need for model testing • geotechnical conditions and geotechnical stability during installation and operation • analyses of all phases during construction and installation, specially emphasising floating stability and requirement for stability in case of damage • designing of all critical sections, with calculation of section forces and moments, including amount of rebars and detailing of the need for pre stressing (see Chapter and Chapter 5) For structures of the Condeep type, the geometry in the tri-cell areas and other joints are to be designed in detail • element analyses of critical sections like top of shaft (see Chapter 4) • the safety of the platform’s mechanical systems e.g ballast systems and the oil company’s mechanical systems (flowlines from wells, risers, oil storage system etc.) • choice of bulkhead sectioning and analyses of floating stability during operation For floating platforms, specially emphasising how the platform can be designed with acceptable spare buoyancy to allow for some topside load increases in the later phases For fixed and floating structures, respectively, the following calculations shall be discussed in depth: Fixed concrete structures: • Floating stability, including level of ballast water in all essential stages of fabrication • Global wave forces, including the requirements for calculation of forces in the shaft base (Morison wave load analysis with Stokes 5th order wave theory) © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin • Dynamic calculations, including demonstration of the natural response period if “springing” and “ringing” (transient dynamic response) can give a substantial contribution to the global wave forces • Geotechnical evaluation, both of geotechnical stability as well as of soil structure interaction in conjunction with dynamic calculations, including analysis of soil pressure distribution under the foundation during various load applications Furthermore the possibility of soil erosion shall be assessed Floating concrete structures: • Floating stability, including estimate of the level of ballast for all essential stages of fabrication and operation • Detailed hydrodynamic calculations of wave forces as well as global responses In addition, it should be noted that certain stages in connection with fabrication can lead to special design forces in parts of the structure • Dynamic design, including identification of the various natural response periods where “springing/ringing” can contribute to a substantial part of the global wave forces • Geotechnical calculations in connection with the mooring systems • Analyses of the water tightness of the structure to assure that the structure can be safely operated throughout its entire design life The oil company normally gets most of this work performed by experienced consultants/ contractors Concerning development of offshore concrete structures, it is essential that the contractors tendering for construction are allowed to develop competitive structures adopted to their own construction facilities Thus, the oil company can obtain competitive bids from several competent tenderers 2.7.3 Quality assurance and verification It is assumed that the work performed during the concept development phase has been subject to the proper quality assurance, see Appendix A (Discipline Activity Model) The principles described here are used both by the oil company and the external contractors involved In addition it is relevant to use external consultants to perform verification of parts of the documentation For this phase it is recommended to channel all communication related to verification through the oil company (see Chapter 7) There is also a need for a rough risk analyses during the concept definition phase to identify whether the concept has built in major risks during the phases of construction or operation related to: • loss of human life • damage to the environment • loss of values Thus, risk reducing efforts can be incorporated Special emphasis should be made to focus on the following risk elements during the construction phase: © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin • • • • risk of falling objects risk of uncontrolled ingress of water due to pipe failure or malfunctions of the ballast system risk of collisions risks during towing and installation In addition, the accidental design loads applied in the operation phase are to be determined 2.7.4 Schedules and budgets An important part of the concept definition phase is to establish detailed schedules and budgets for the development project 2.7.5 Competence requirements The concept report represents the final basis for a successful performance of a development project Consequently, it is of the highest importance that all parties involved use their most experienced engineers and make them responsible for the project’s concept evaluation phase 2.8 Project organization phase 2.8.1 Introduction The project Organisation Phase represents an intermediate phase inbetween the phases of concept definition and construction Note that detail design is part of the construction phase During the project organisation phase the construction work (fabrication) is prepared Several fundamental principles of technical nature have therefore to be clarified during this phase 2.8.2 Contract A contract for the construction phase has to be established in this phase Included herein is choice of type of contract The following definitions can be used for important tasks during the construction phase: E P C I Engineering work (detail design) Procurement Construction Installation The contract for construction can include one or more of these tasks For construction of offshore concrete structures the normal contract form has been to select one contractor for the whole work, i.e an EPCI-contract In some cases, though, the oil company has separated the © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin Main Mechanical Outfitting work (MMO-work) from this contract and placed that part of the scope with a separate mechanical contractor This split of contract implies that the oil company has to co-ordinate the concrete and mechanical contractors In other cases the detail design is also assigned to a separate design contractor The advantage is that more contractors can tender for construction after the completion of the detail design work The drawback is that this leaves the oil company as responsible for assuring that the detail engineering is performed properly and that the design is constructable, and issued in due course Any changes may lead to claims for costly change orders Most of the arguments lead to the preference of one EPCI-contract for detailed design and construction of an offshore concrete structure Such a contract puts the responsibility on one contractor The oil company has in advance qualified the tendering contractors The judgement regarding qualifications are based on technical competence, experience and financial strength Of special importance is to have documented that the project can be performed according to schedule With such a comprehensive contract format as described above, it is of paramount importance that the contract clarifies essential relations between the oil company and the contractor This applies to inter alia: • list of specifications according to which the work shall be performed • required detailing of contractor’s documentation and work, including detailed specifications of the basis for the work and requirements for co-operation with the oil company Especially, it is regarded important that the oil company has the right to take direct control over the amount of reinforcement (payment by unit rate) • requirements for efficient organisation of contractor’s work, to ensure that safety, environment, cost and progress are secured Included in this is clear definitions of “who to what” in contractor’s organisation, and of which subcontractors be accepted and how they are managed • requirements to contractor’s technical know-how, including requirements to the involved persons Special requirements must be set to contractor’s technical management and their co-ordination of the technical work • requirements to the contractor’s internal control • requirements to co-operation between the contractor, the oil company and the consultant performing the third party verification, including agreement with the contractor as to how comments from the third part verification shall be duly incorporated • procedures for reporting of results, progress, problems and deviations • agreement of milestones for revisions, and agreement of the oil company’s right to claim revisions as and when it finds it necessary This also applies to revisions of subcontractors • agreement on commercial relations Special emphasis is to be made such that the contractor shall follow the Concept Report issued by the oil company during the concept definition phase It is assumed that the oil company has established a thorough concept report which includes: • robustness to withstand minor changes in the design basis, for instance increased weight of the topsides within ±5% • “forgiveness” to avoid major changes to the concept, if minor omissions are made in the concept definition phase which lead to changes in dimensions and weight © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin This does, however, not mean that safety margins shall be applied beyond the requirements of the national rules and standards Arrangements must be laid down in the contract to ensure that all changes to the concept report shall be agreed by the oil company and treated according to specially agreed procedures During contract preparation and negotiations it is important that the oil company’s project group has high technical expertise and long administrative experience 2.8.3 Quality assurance plan During the Project Organisation Phase the oil company must prepare a plan for how to perform quality assurance of the work during detail engineering and construction As to procedures and some available tools for quality assurance, see Chapter 2.8.4 Verification plan A plan for verification has to be developed The plan shall include: • description of the extent of work for the verification, emphasising special critical areas of the structure • requirements to the oil company’s own verification of contractor’s work, with description of the follow-up work during the detail design and construction activities • requirements to the contractors’ internal quality control and surveillance • working methods and work description valid for the consultant performing the third party verification • requirements for prompt implementation of the results from the verification The choice of strategy for external verification is of special importance The oil company must have direct contact with the third party verification consultant Payment should be made according to spent hours within agreed limitations Chapter details two models for external verification: the oil company submits the results from the verifying consultant to the contractor company is managing the verification, but the verification consultant communicates mainly directly with the contractor Regardless of which model is followed, the verifying consultant is obliged to follow a tight schedule not to delay the progress of the contractor The oil company must take all formal decisions in case disagreements arise between the contractor and the third party verificator The oil company should also be the single point contact with the authorities and the partners © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin 2.8.5 Preparation for the organization of the construction phase During the process of preparing for a development project, project organisations are built up within the oil company and the contractor The oil company is to ensure that both his own personnel and those of the contractor have the acceptable competence This is partly done by specifying competence requirements for every leading position in the project Every leader must have thorough knowledge of the technical content of the activity he is meant to lead It is presupposed that the project is organised to assure openness with respect to technical questions, and that the requirements for quality in performance is characterising the dialogues and feed backs No project philosophy which suppresses technical problems should be allowed In offshore field development projects, key persons who have worked during earlier phases in the same project should be brought in, so that continuity in the technical work is maintained All staff in a project must agree to the project’s goals and the procedures applied to reach these goals References Fjeld, S and Morely, C.T (1983) Offshore Concrete Structures, in Handbook of Structural Concrete Eds Kong, F.K., Evans, R.H., Cohen, E and Roll, R, McGraw Hill Norwegian Council for Building Standardisation, NBR (1999) Specification texts for building and construction, NS 3420, Oslo, Norway, 2nd edition 1986, 3rd edition 1999 Norwegian Council for Building Standardisation, NBR (1998), Concrete Structures, Design rules NS 3473, 4th edition, Oslo, Norway, 1992 (in English), 5th edition 1998 (English edition in print) Norwegian Petroleum Directorate, NPD, Rules and Regulations for Petroleum Activities, New edition issued every year by the Norwegian Petroleum Directorate, Stavanger, Norway Norwegian Petroleum Directorate, NPD (1992) Regulations relating to loadbearing structures in the petroleum activities, stipulated by the Norwegian Petroleum Directorate, Stavanger, Norway Tjelta, T.I., Aas, P.M., Hermstad, J and Andenæs, E (1990) The skirt piled Gullfaks C platform installation Paper OTC 6473 Proceedings Offshore Technology Conference, pp 453–462, Houston, Texas © 2000 Edited by Ivar Holand, Ove T Gudmestad and Erik Jersin ... phases of the development Appendix A (Discipline Activity Model for Design of Offshore Concrete Structures) indicates the level of detail for technical studies relating to an offshore concrete. .. Jersin 2.6.2 Selection of concrete structures Concrete structures are relevant for quite a number of various field developments It should be noted that: • fixed concrete structures with short... simplified by the use of PC-based tools An illustration of input data needed is shown in Fig 2.17, see also Appendix A (Discipline Activity Model for Design of Offshore Concrete Structures) In the